STUDIES OF METEORITE CRATERS 2.1. Meteorite craters worldwide and in Estonia
Most of the known 172 recognized hypervelocity meteorite impact sites on Earth are located onland (c. 150 million km2) (Grieve 1987; Grieve et al. 1995;
Grieve & Pesonen 1996; Gilmour & Koeberl 2000; Earth Impact Database (http://www.unb.passc/Impact). Yearly 3–5 new discoveries are added into the global crater record. Compared with other rocky planets or their natural satel-lites in the solar system this is a relatively small number. On the Moon, alone, there are identified more than 20 000 meteorite craters. The thick atmosphere and the large hydrosphere that covers 70.8% of the Earth surface, as well as the activity of geological processes has resulted in the low number of recognized impact structures on Earth.
Due to the active geological processes on Earth, even in the case of survival, most of the impact structures are soon heavily destroyed or hidden below sedi-mentary suites or water. Therefore, the recognition of impact structures requires some enthusiasm in observations and the application of special investigations.
Presently, the average cratering rate on Earth is around 1 per c. 3 million km2. In Fennoscandia, within an area of 3 million km2 onland and offshore, about 60 proved and possible impact structures were discussed in 1991 (Henkel 1992;
Pesonen & Henkel 1992). Due to the many specific field and laboratory studies, the number of identified craters in Fennoscandia reached 30 in 2005 (Abels et al. 2002; Puura & Plado 2005). Thus, cratering rate in Scandinavia reached 1 per c. 100 000 km2.
The Estonian mainland is 45 277 km2 and the aquatory is c. 75 000 km2 which amounts to ca 0.015% of Earth. In 1970, the local list comprised four impact sites, including two crater fields (Kaali and Ilumetsa), two single craters (Tsõõrikmäe, Simuna) and two traces of meteorite impact (Lasnamäe, Vaida-soo). Thus, the cratering rate in Estonia is 1 per c. 10 000 km2, which is about 300 times higher than on the Earth in average, and about 10 times more than in Fennoscandia. However, six of the structures in this list (Kaali, Ilumetsa, Tsõõrikmäe, Simuna, Vaidasoo and Lasnamäe) are small (less than 1 km in diameter) and less than 1 million years old. The lack of small (less than 1 km in diameter) and young craters in the cratering record of other countries is not easy to explain. Two explanations may be valid: a) adverse conditions of preserva-tion of impact structures, b) insufficient search for small meteorite structures.
Although E. Öpik (1916) was one of initiators of the physical theory of meteor phenomena (impact theory), he was unsuccessful in discovering any craters in his homeland (Öpik 1916, 1936, 1958). Since 1922, when J. Kalkun suggested that the crater-shaped Lake Kaali on Saaremaa Island was of a
meteorite impact origin (Kalkun 1922), the geologists in Estonia have paid a lot of attention to possible impact sites. Starting a new cycle of field studies in 1927, I. Reinwald could already in 1937 prove the meteorite origin of the Kaali crater field. He found meteorite iron in a satellite crater (Reinwald 1937, 1938).
Afterwards, many new small and two larger complex craters were found in Estonia (Tiirmaa 1997; Suuroja & Suuroja 2004; Tiirmaa et al. 2006).
The main types of impact structures on the Earth are (a) bowl-shaped simple craters (up to 3–4 km in diameter), (b) complex craters – bowl-shaped craters with central uplift (from 3–4 to 10–20 km in diameter) and (c) large multiring basins (with diameters to some 300 km). The Kärdla structure (4 km in dia-meter) is a typical complex bowl-shaped crater with central uplift. In the setting of the Neugrund structure (20 km in diameter) distinct features of multiring kind occur.
2.2. The Kärdla crater
Throughout the history of geological studies in the northeastern part of the Hiiumaa Island, many unexpected, unusual and unexplained signatures were revealed, which were understood only after the discovery of the Kärdla meteorite crater. Bedrock dislocations, namely tilted bed of limestone, along the slopes of the Paluküla Hill were described already in the 19th century (Eichwald 1840; Ozersky 1844; Schrenk 1854; Schmidt 1858). The dislocations were later identified as the buried rim wall of the Kärdla crater. Later, data on the occur-rence of asphalt (Winkler 1922; Scupin, 1927) and galena (Palmre 1961) at Paluküla were published. In 1966, drilling of an artesian well on the Paluküla Hill was stopped, because rocks of the Precambrian crystalline basement were unexpectedly met at the depth of 16 m, instead of anticipated c. 230 m. The revealed uplift of crystalline basement rocks was additionally drilled and interpreted as a placanticlinal (Viiding et al. 1969). A nearby structural depres-sion, which was revealed in course of drilling for geological mapping, was first explained as a tectonic graben, and the impact breccias penetrated in drill hole 412 were interpreted as late Precambrian tillites (Kala et al. 1971). The depres-sion was later found to be the central uplift of the crater proper. During search for uplifts of granitic rocks, suitable for producing high quality splinter, detailed geophysical mapping by means of gravimetry and magnetometry) was carried out (Barankina & Gromov 1973). Tens of prospecting wells were drilled in the area (Suuroja et al. 1974). As a result, the crater structure was revealed (Puura 1974) and a hypothesis of its crypto-volcanic origin of it was put forward (Suuroja et al. 1974). During subsequent prospecting (Kala et al.1974, 1976) the hypothetic crater floor was reached at the depth of c. 500 m (drill hole K-12).
The thin sections of matrix-supported breccias from drill core K-12 were studied in late 1970-s in the laboratory VSEGEI, St. Peterburg (then Leningrad) and PDF-s (planar deformations features) were found in quartz and kink bands
in biotite. Using these data Masaitis et al. (1980) distinguished and published the impact origin of the crater.
The discovery of the Kärdla crater caused initiated a number of large pro-jects focused on detailed geological mapping and deep drillings within the structure and in its surroundings (Suuroja et al. 1981, 1991, 1994, 1997).
Prospecting for mineral resources also supported the geological investigations.
In the course of prospecting for mineral water (Tassa & Perens 1984), the well K-18 in the central part of the Kärdla crater depression was drilled and more than 130 m high central uplift (peak) was discovered (PAPER I; Puura &
Suuroja 1984, 1992). For the geological mapping, the deepest (815.2 m) well in Estonia, Soovälja K-1, was drilled within the annular depression of the crater.
The core represents the most complete sequence of the main lithologies of the crater infill and, starting from the depth of 523 m, of the impact-dislocated sub-crater basement (Fig. 1). A large number of the cores were drilled in the sub-crater interior. Among them 6 wells reached the sub-crater basement. Along the crater rim wall, 46 wells reached the impact breccias and the crystalline basement rocks. In the crater exteriors, 64 wells reached the distal ejecta blanket or dislocated pre-impact target rocks. The total number of drill holes giving original information on the crater structure and covering deposits reached 162.
Numerous drillings, geophysical and other materials became considerable source for scientific research. In 2005, the total number of published research papers on the Kärdla structure reached around 50. Since 1973, the author of the present thesis has concentrated on the mapping and investigation of the geology, formation and development of the Kärdla structure. Geophysical-maps, core logs, structural sketches and subsurface geological maps at different structural levels, cross sections summarizing the existing materials served as basis for selecting new drilling sites. The core studies, which were mainly carried out by the author, have included observations and descriptions of core samples and interpretation of geophysical logging data. Large numbers of samples were collected and sent to laboratories for petrographic, lithological, mineralogical and geochemical analysis. The results are presented and used in the published papers. Initially, the main parameters and main features of the crater were presented in the early papers (PAPER I; Puura & Suuroja 1984, 1992). Subsequently, the main results were presented in papers III–IV. Apart from in the present thesis, crater data has been presented in many additional papers by the author, his co-authors and researchers involved into the study of the drilling cores and other materials. In 1996, in co-operation between the Stockholm University and the Geological Survey of Estonia, seabed off from the northeastern margin of Kärdla crater was studied from board of the research vessel “Strombus”. Continuous seismic reflection profiling proved the existence of the presumed ring fault around the crater in Kärdla Bay (PAPER IV; Suuroja et al. 2002).
2.3. The Neugrund crater
Also the Neugrund structure was discovered due to attention paid to indirect phenomena. In the course of geological mapping of northwestern Estonia (Kala et al. 1969) disturbances in the sequence of Ediacaran (Vendian) and Early Cambrian clays and sandstones were observed in drill core 410 on Osmussaar Island. At the time there was no explanation to these structures. Again, almost two decades later, in the course of deep geological mapping of North-western Estonia, in the drill cores (F-331, F-331A, F-332, F-335) intervals of brecciated clay- and sandstones of the upper part of the Early Cambrian subsequences were observed (Suuroja et al. 1987).
Much earlier, in the same coastal area of North-western Estonia, A. Öpik observed, and N. Thamm described, large erratic boulders of an odd gneiss-breccia along the seashore (Öpik 1927, Thamm1933). Also K. Orviku and H. Viiding described these erratic boulders, but nobody could explain their origin (Orviku 1935, Viiding 1955)). Revisiting the area in the course of the geological mapping K. Suuroja and T. Saadre noted that these gneiss-breccias are macroscopically very similar to clast-supported impact breccias from the Kärdla crater (Suuroja & Saadre 1995; Suuroja et al. 1998). They suggested that the specific gneiss breccias of North-western Estonia are impact-produced breccias possibly drifted by glacier. A specific pattern in the seabed, namely the nearby Neugrund Bank in southwestern part of the Gulf of Finland, was proposed as the possible impact structure. Previously, in the course of marine geological investigations (Malkov et al. 1986; Lutt & Raukas 1993; Talpas et al.
1993), a ring-shaped wall around the Neugrund Bank was observed, but it was interpreted as a glacial moraine wall.
In co-operation with the Geology Department at the Stockholm University, and using their equipment and methods, the seabed in the surroundings of the Neugrund Bank was studied by seismic reflection profiling from the Stockholm University research vessel Strombus in 1996 (see method descriptions in Flodén 1980, 1981). As the result of these investigations, a 9-km diameter crater-like structure, surrounded by 4–5 km wide zone of dislocations, was discovered (Suuroja 1996b). Shock metamorphic minerals (quartz grains with PDF-s) from the erratic blocks of gneiss-breccias were found and the impact origin looked more probable (Suuroja et al. 1997). In 1998, a diving geologist took samples from submarine outcrops along the rim wall composed of Precambrian meta-morphic rocks. Samples similar to the breccias from the erratic boulders yielded quartz grains with PDF-s and other signs of shock metamorphism. The impact origin of the Neugrund structure was proved (PAPERS II, V, VI; Suuroja &
Suuroja 1999, 2000; Suuroja et al. 1999b, 2003).
In the period 1998–2003, 7 marine expeditions were carried out on the Neugrund impact structure area. The vessel Mare of the Estonian Maritime Museum was used. Captain Vello Mäss directed the marine and submarine investigations, including diving. The total number of diving sites was 21,
located at depths between 2–42 m. At 12 sites the divers succeeded in collecting samples, 0.1–3 kg in size, of impact-produced as well as covering rocks. With few expeditions, the diving’s were recorded by a camcorder accommodated with ikelite underwater systems. Additionally, the submarine outcrops were observed by a camera system SeaLion mounted on a remote operated vehicle (ROV) and side-scan sonar (SSS) profiling.
Apart from sampling, the diving geologists made descriptions of each sampling site. An assistant diver using a digital depth gauge measured the depth of each sample. The depth, with accuracy 0.1 m, was noted on each sample using a waterproof pencil. Concomitantly, another assistant diver recorded the outcrop and sampling site to videotape. The sampling sites are located within the crater infill, which is exposed on the central plateau of the shoal.
Additionally, exposures of impact influenced Precambrian basement meta-morphic rocks were observed and sampled on the ring ridges.